1. Field of the Invention
The present invention concerns a static kWh meter, comprising members connected to the leads of the distribution network for producing measuring signals proportional to the current and the voltage, a pulse width-pulse height multiplicator for forming a signal representing the product of the measuring signals, a frequency/voltage converter with an analog integrator integrating the multiplicator output, for generating a frequency signal proportional to the power, and a counting device controlled by the frequency signal for determining the energy consumed, and a power pack unit connected to the mains leads.
2. Description of Related Art
Static kWh meters based on a pulse width-pulse height multiplicator, i.e., on a time-chopping multiplicator, are commonly known in the art, and as examples of existing design solutions may be mentioned the U.S. Pat. Nos. 4,315,212 and 3,794,917.
The main components of a three-phase meter of this type known in prior art are presented in the block diagram of FIG. 1. In the meter, a sample pulse sequence is formed in which the width of the pulses is proportional to the momentary value of the voltage and the height of the pulses is proportional to the momentary value of the current, so that the area subtended by the pulses is proportional to the product of voltage and current, i.e., to the power. The sample pulse frequency is selected to be approximately ten times higher than the mains frequency in order to account for superharmonics as well. The signal modulating the pulse width modulator 1, is a signal proportional to the mains voltage, produced by the aid of voltage transformers 2. In meters known in the art, the pulse width modulation has usually been carried out by means of an operational amplifier acting as a comparator, to one input terminal thereof being conducted a triangular wave and to the other input terminal a modulating signal proportional to the voltage. The threshold level of the comparator is controlled by the modulating voltage, and at the output of the comparator is obtained a square wave having the same frequency as the triangular wave but dependent on the value of the control voltage, as regards its pulse proportions. By the pulse width-modulated square wave obtained are controlled analog switches 4 in the secondary circuits of the current measuring transformers 3, from their outputs being obtained pulses with a height proportional to the current in the measuring circuit, and which are still modulated by the mains voltage as regards their width. The pulses are filtered so as to obtain a d.c. voltage proportional to the power, and this voltage is supplied to a voltage/frequency converter 5. The voltage/frequency converter 5 consists of an integrator amplifier integrating the output of the multiplicator and a threshold level detector, which initiates a new cycle when the output of the integrator exceeds the threshold level of the detector. Thus, the output of the converter delivers a pulse frequency which is proportional to the d.c. voltage supplied thereto, that is to the power, and by which the step motor actuating the counter means 7 of the meter is controlled over a divider unit 6. The counter means integrates the power, in other words, it displays the energy consumed. For the power supply of the circuits of the meter, a power pack 8 is moreover provided, producing the operating voltage from the mains voltage.
Static kWh meter designs of prior art are comparatively complex, and the units of the meters contain operational amplifiers, of which the offset voltages exert an influence on the accuracy of the meter.